Electrodynamic machines convert mechanical energy to electrical energy (generator mode) or electrical energy to mechanical energy (electric-motor mode). The conversion is based on the Lorentz force, which acts on moving changes in a magnetic field.
By way of example, but without imposing any restriction, the following text relates to electrodynamic machines which operate as generators. In particular, the statements relate to large-technical installations, for example large synchronous generators, such as those used for industrial electricity generation.
The generator has a rotor which is mounted on a shaft and rotates at a rotor rotation frequency within a stationary stator. During rotation, the rotor produces a revolving magnetic constant field, which induces a sinusoidal electrical voltage, and therefore a sinusoidal current, in the stator windings. The constant field of the rotor is produced by windings through which current flows, and the windings are arranged in slots which run parallel to the rotation axis. In large-technical installations, the windings consist, for example, of hollow metal strips whose outer surfaces are electrically insulated from one another by means of a plastic layer. A cooling medium generally circulates in the interior of the hollow metal strips.
Electrodynamic machines, in particular large generators, are monitored during operation in order in particular to identify vibrations of the shaft or the causes thereof at an early stage, and in order in this way to make it possible to avoid damage to the machine.
The vibrations of the shaft preferably occurs in the form of torsional oscillations, pendulum oscillations and/or bending oscillations as well. Torsional oscillations are oscillations which lead to a non-uniform rotation frequency along the shaft, and therefore to torsion in the shaft. By way of example, they can be caused by sudden load changes. Torsional oscillations are very small oscillations, normally with a phase amplitude in the region of 0.01°, but can nevertheless lead to a very high load on the shaft. The frequencies at which torsional oscillations occur depend on the material character and on the thickness of the shaft, on the masses connected to the shaft, and on the size of the installation.
Pendulum oscillations are quasi-periodically damped changes in the rotation frequency of the shaft about the mains frequency (typically 50 to 60 Hz) which are caused, for example, by disturbances in the form of surges from the mains, and likewise have a torsional effect on the shaft. However, pendulum oscillations typically lead to considerably slower changes in the instantaneous angular velocity at which the shaft rotates, and the instantaneous phase of the rotary movement, than the torsional oscillations which have been mentioned. For example, in the case of a large-technical generator for industrial electrical generation, which has a rated rotor rotation frequency of typically 50 or 60 Hz, torsional oscillations occur at typical frequencies between 100 and 300 Hz, and pendulum oscillations at typical frequencies of 5 to 10 Hz.
Furthermore, vibrations can also be caused by rotor turn shorts or else by non-uniform pole arrangements of the magnetic field produced by the rotor.
In order to protect the electrodynamic machine against damage, it is shut down when major vibrations occur.
U.S. Pat. No. 3,506,914 describes a method by which rotor turn shorts can be detected which, as mentioned above, can lead to vibrations. So-called stray-field measurement is used to detect rotor turn shorts, in that the stray magnetic field which is produced by the rotor and tangentially to the rotor surface with the aid of air-gap sensors, which are fitted between the rotor and the stator in the air gap. This makes use of the fact that turn shorts result in the stray field being modified in a way which can be measured in the air gap.
Another approach to determine shaft oscillations, which is described in various documents, for example U.S. Pat. No. 3,934,459, U.S. Pat. No. 3,885,420, U.S. Pat. No. 4,148,222, U.S. Pat. No. 4,137,780, U.S. Pat. No. 4,317,371, consists in that a toothed wheel is connected to the shaft in order to detect torsional oscillations of the shaft outside the rotor and the stator, which toothed wheel produces periodic measurement signals in one or more circumferential sensors during the rotation of the shaft, with the period depending on the number of teeth and the rotation frequency of the shaft. Torsional oscillations of the shaft result in phase or frequency modulation of the detected signals. However, this procedure has the disadvantage that a tooth wheel must be provided as a signal transmitter. Retrospective installation may be impossible or difficult, and may be associated with correspondingly high costs.
One method for detection of torsional oscillations, described in U.S. Pat. No. 4,444,064, consists in first of all applying a magnetic pattern to the shaft, which acts as a pulse transmitter during operation.
U.S. Pat. No. 4,793,186 describes a measurement instrument for torsion measurement, which is connected to the phase winding terminals of a permanent-magnet generator which is coupled to the shaft of a generator. The torsional oscillations are deduced by evaluation, that is to say by frequency demodulation by means of phase locked loop technology, of the voltages which are produced. The method can be used only for systems with a permanent-magnet generator, and, furthermore, is relatively inaccurate.
A further possible way to detect torsional oscillations along a generator shaft is described in JP 52110449. This is based on the measurement and evaluation of phase terminal currents in the generator. This method is also relatively inaccurate and, furthermore, requires current transformers for relatively high frequencies.
The methods mentioned above relate exclusively to the measurement of torsional oscillations, but not to the detection of bending oscillations.
In contrast, EP 1 537 390 B1 describes a method by means of which not only torsional oscillations but also pendulum and bending oscillations can be detected. For this purpose, shaft currents and/or shaft voltages which occur on the shaft are tapped off and evaluated outside the generator.